AccountSettings: escape forward- and backslashes in MXIDs

[KitsuneRal]

Fixes #842.

[KitsuneRal]

The test is screwed (and should really be a unit test, not a part of quotest) but the functionality seems to be working, as just tried in Quaternion.

[TobiasFella]

This makes the account load fine, but it's not yet perfect: my test account with mxid @tobias/:matrix.org is shown as @tobias%2F:matrix.org.

(some other things are also broken, but I'd look at those in different issues / patches) actually those might just be symptoms of that problem, let's see

[KitsuneRal]

We could add unescaping in some other places; I'm just trying to stay conservative. Technically, it's possible to recover MXIDs even at the Settings level, by parsing the key and only (un)escaping only the piece that looks like an MXID - e.g., by using the fact that nobody's using : in key names so far, outside of the account names.

[KitsuneRal]

Looks like the Windows runner doesn't allow me to store settings in the registry?..

[KitsuneRal]

This makes the account load fine, but it's not yet perfect: my test account with mxid @tobias/:matrix.org is shown as @tobias%2F:matrix.org.

(some other things are also broken, but I'd look at those in different issues / patches) actually those might just be symptoms of that problem, let's see

Okay, I found why this happens, AccountSettings::userId() has to unescape the returned value too. Fix coming.

[KitsuneRal]

No idea what is happening on Windows CI: it successfully create a settings group for the account but fails to save anything inside the group. Given that it's failing even with "normal" MXIDs, I tend to blame the GitHub runner. Feel free to test it out, if it's fine, I'm merging it.

[KitsuneRal]

I figured the problem on Windows in the end - QSettings really wants the organisation name to be explicitly set there, the default empty organisation name doesn't turn into Unknown organization as it does on Linux and an empty name for the the registry key doesn't work, of course.